Detection system employing digital bandpass filtering circuitry

ABSTRACT

An acoustic detection system of the Doppler variety is provided with digital filtering circuitry for eliminating the false alarming effects of various spurious sources which are situated in or near a space being monitored for a predetermined type of motion. Squaring circuitry is provided for converting the normally analog waveforms to digital waveforms whereby simple digital band-pass filters may be used to sharply discriminate against those frequencies considered non-attributable to the particular motion of interest.

United States Patent Perlman et al.

[ Aug. 1,1972

[54] DETECTION SYSTEM EMPLOYING DIGITAL BANDPASS FILTERING CIRCUITRY[72] Inventors: David E. Perlman, 59 Stoneham Drive, Rochester, N.Y.l4625; Donald S. Degen, 124 Yarmouth Road, Rochester, NY. 14610 [22]Filed: March 19, 1970 [21] App]. No.: 20,887

[52] U.S. Cl ..340/l R, 340/3 D, 343/7.7 [5 1] Int. Cl ..G0ls 9/66 [58]Field of Search..340/l, 3, 3 D, 3 FM; 343/5 DP, 343/5 PD, 7.7 [56]References Cited UNITED STATES PATENTS 3,394,342 7/1968 Walker ..340/lRECE'VER I60 I6]: I70

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9. {OSCILLATOR PHASE RATE DETECTOR Primary Examiner-Richard A. FarleyAttorney-Warren W. Kurz [57] ABSTRACT 10 Claim, 11 Drawing FiguresDIGITAL FILTER fl "I HIGH-PASS FILTER I LOW-PASS l FILTER RELAY DRIVERPATENTEmuc I \912 3.681. 745

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C6 5, iy-me Q7 C7 C8 RIB FROM 05 1 PHASE 06 DETECTOR G4 I v P 03 Q4 05R5 I K R2| l J A 3 c3 02% R9 Rum K Rl3 R's m? 09 21 AVERAGING AUDIO F 5REGENERATIVE CIRCUIT AMPLIFIER SQUARING CIRCUIT FIGS/l m Q6 I E Fla-6BCOLLECTOR Q1 N FIG. 66

INVENTORS DAVID E. PERLMAN DONALD s. DEGEN av z/ AGENT PATENTEDIIIII: I\972 3.681.745

SHEET3UF4 TO DIGITAL FILTER V+ l L l /Rl2 gRI IL C6 g, RIe FROM ,cs C7rl PHASE II DETECTOR G4 I l 03 Q4 [2Q5 R5 J R2|\ (:3 RF R9 RIM? R|3 1 7R2x AVERAGING AUDIO F [6 5 REGENERATIVE CIRCUIT AMPLIFIER SQUARINGCIRCUIT BASE Q6 THRESHOLD O FIG-5B INVENTORS DAVID E. PERLMAN DONALD s.DEGEN mr z/mmviy AGENT DETECTION SYSTEM EMPLOYING DIGITAL BANDPASSFILTERING CIRCUITRY BACKGROUND OF THE INVENTION systems wherebyelectrical signals generated by false I alarm-producing sources may bedistinguished from those generated by the occurrence of a predeterminedtype of motion, and thereby be prevented from producing a false alarmingof the system.

In acoustic or ultrasonic detection systems of the Doppler variety, suchas disclosed in US. Pat. No. 2,794,974 to S. M. Bagno et al., it iscommon to subject the output signal of the Doppler frequency detectioncomponent (i.e., the mixing circuit) to bandpass filtering beforetransmitting it to the alarm activating component so as to sound analarm. Such filtering serves to prevent certain undesirable frequencycomponents (i.e., those frequencies which are considered either too highor too low to be considered attributable to the specific motion ofinterest) from reaching the alarm component and thereby generating falsealarms. In conventional systems, such filtering is accomplished by theuse of RC networks. While extremely sharp bandpass filtering may beaccomplished by cascading stage after stage of RC filtering networks,such filtering requires the use of a multitude of circuit elements andthus tends to be costly. Moreover, RC filtering networks may be subjectto undesirable ringing and may not attenuate sufficiently outside of thepass band to block very large spurious signals.

SUMMARY OF THE INVENTION Accordingly, it is the primary object of thepresent invention to provide an improved Doppler detection systemcomprising extremely sharp bandpass filtering circuitry which suffersnone of the aforementioned disadvantages of RC filtering circuitry. Inaccordance with a preferred embodiment, this objective is accomplishedby the provision of a detection system which includes a motion detectingcomponent that is capable of generating a digital signal comprising atrain of equal amplitude rectangular pulses, each pulse having a widthproportional to the average rate of motion of objects within the spacemonitored by the system at the time the pulse is generated, and digitalfiltering circuitry whereby pulses having pulse widths and frequency ofoccurrence below a predetermined minimum value may be filtered out ofthe output of the motion detecting component. Means are provided forintegrating the signal transmitted by the filtering circuitry and forproviding an alarm activating signal when the integrated signal strengthexceeds a predetermined threshold within a predetermined time. Thedigital filtering circuitry has the advantage over RC circuitry of beingable to provide superior filtering quality with much fewer electricalcomponents. Moreover, such circuitry is not subject to ringingoscillation. Other objects and advantages will become apparent to thoseskilled in the art upon reading the detailed description of a preferredembodiment provided below.

BRIEF DESCRIPTION OF THE DRAWINGS In the detailed description of thepreferred embodiment of the invention presented below, reference is madeto the accompanying drawings, in which:

FIG. -1 represents, in block diagram form, an acoustic detection systemembodying the present invention;

FIG. 2 is a schematic circuit diagram illustrating a preferred circuitof a logical NOR gate for comparing 0 the phase of two waves;

FIG. 3 depicts a truth table for the NOR gate illustrated in FIG. 2;

FIG. 4 shows the manner in which the circuit illustrated in FIG. 2responds to various inputs;

FIG. 5 is a schematic circuit diagram showing preferred circuitry forthe averaging, audio amplifying the regenerative squaring circuitscomprising the system shown in FIG. 1;

FIG. 6 illustrates the signal waveform at various points in the circuitdepicted in FIG. 5;

FIG. 7 is a schematic circuit diagram showing a preferred form of thedigital filtering circuit comprising the invention; and

FIGS. 8 and 9 illustrate possible signal waveforms at harious points inthe digital filtering circuit depicted in DETAILED DESCRIPTION OF THEPREFERRED EMBODIMENTS Referring now to FIG. 1, an ultrasonic detectionsystem embodying the invention is shown in block diagram form. While thesystem shown operates on the well-known Doppler principle, it is of anon-conventional variety in that it relies on circuitry for sensingchanges in phase between the transmitted and received acoustic waveswhich the intruder produces by his movements in the protected area,rather than circuitry for sensing changes in frequency. It will becomeapparent, however, from the ensuing description that the invention hasequal utility in the more conventional frequency sensing system.

A continuous acoustic wave, preferably ultrasonic and having a frequencyof approximately 40,000 cycles/second, is transmitted into the area tobe protected from intrusion by a conventional electro-acousticaltransducer 10. Transducer 10 is driven by the output a of an oscillator11 of standard design. The identical signal which drives the transducer10 is also fed to a standard limiting circuit 12 which serves to convertthe sinusoidal output of the oscillator to a square wave. The squarewave output 0 of limiter 12 is then fed directly to one of the twoinputs of phase comparator 13, forming a part of a phase rate detector14. Both phase comparator l3 and rate detector 14 are described indetail hereinafter.

Upon being reflected by objects in the protected area the ultrasonicwave is received by a receiving element 15, such as a crystal microphonewhich reconverts the acoustic energy to electrical energy. According tothe Doppler effect, the received wave will be a sinusoidal waveformhaving a frequency dependent upon the rate of change in position of anyof the objects in the protected area. When there is no motion in thisarea, the

frequency of the received wave, of course, is identical with that of thetransmitted wave. The normally lowlevel sinusoidal output b of receiveris then amplified and limited by a series of amplifiers 16a, 16b and 16cand limiters 17a and 17b, all being of conventional design. Amplifier16c is fairly sharply tuned so as to exclude unwanted ultrasonicfrequencies from noise sources such as hammering radiators and to obtainan increased phase shift due to frequency changes arising from movingtargets; i.e., the Doppler effect. The purpose of the limiting circuitsis to permit phase comparison that is independent of amplitude. Therelatively high level square wave output 0' of the final limiting stage17b is then fed to the remaining input to the phase comparator 13.

Phase comparator 13 is of the digital type which is capable of providingat its output, A0, a train of pules, each having a fixed amplitude and apulse width proportional to the instantaneous phase difference betweenthe transmitted and received waves; i.e., the difference in phasebetween the two square wave inputs as provided by limiters l2 and 17b.The frequency of the pulses constituting output A0 is, of course,determined by the frequency of the reference input signal 0 which, inturn, is governed by the output frequency of oscillator 11. Thus, thefrequency of A0 is ultrasonic (e.g., 40,000 cycles/sec.

The output A0 of phase comparator 13 is then passed through an averagingcircuit 18 which serves to convert this digital output to a fluctuatingvoltage (analog signal) having a frequency equal to the instantaneousrate of change of phase. Since the rate of phase change is proportionalto the Doppler frequency shift of the received ultrasonic wave, andsince this frequency shift is proportional to the nature of the motionin the protected area, the output of averaging circuit 18 will becharacteristic of the nature of the motion of the protected area. Byfiltering this output in the manner hereinafter described, thefalse-alarming effects of many spurious sources may be avoided.

To subject the output of the averaging circuit to the digital filteringtechniques of the instant invention, it is first necessary to convertthe analog output of averaging circuit 18 to a digital waveform. Toaccomplish this conversion, the averaging circuit output is first fedthrough a standard audio amplifier 19 which serves to increase signallevel.

Bandpass RC filtering of conventional design is utilized in the audioamplifier to prevent small target-related signals from being masked bylarge disturbances having frequencies outside of the pass band. Theoutput of amplifier 19 is then fed to a regenerative squaring circuit20, preferably of the well known Schmitt trigger variety. The Schmitttrigger, provides a train of pulses of fixed amplitude and of a widthequal to the time during which its input (the audio amplifier output)exceeds the triggering threshold of the trigger circuit.

The output of the Schmitt trigger 20, being a digital waveform, iscapable of being digitally filtered and is fed to the input of digitalfilter 21. Undesirable high frequencies, generated primarily byultrasonic and electrical transients, are filtered out by a low-passfilter 22 which functions by rejecting all pulses having a pulse widthless than a predetermined value. Thus, lowpass filtering is reduced to asimple pulse width discrimination.

The output of low-pass filter 22 serves as the input to high-pass filter23 which responds to the average number of pulses transmitted per unittime by the lowpass filter. Filter 23 will only provide an output if thenumber of pulses within a given time interval exceeds a pre-set minimum.Thus, high-pass filtering is reduced to a simple counting operation.Whenever the pulse input to digital filter 21 has a repetition withinthe bandpass of the digital filter, then an output will be transmittedto the relay driver circuit 24, thereby actuating an alarm.

In FIG. 2, a preferred circuit for accomplishing the aforedescribedfunction of the phase comparator is shown schematically. The circuit isenergized by voltage source v. The square wave outputs 0 and 0 oflimiters l2 and 17 are applied to the bases of transistor 01 and Q2,respectively. When either base is positive, the voltage at the commoncollector point will be low, since Q1 and Q2 by themselves are capableof conducting current through resistor R3. The output will only be high(at supply voltage) when Q1 and Q2 are both 011', a condition occurringwhen their respective bases are at a low potential. The circuit of FIG.2 is essentially a logical NOR gate having a truth table as shown inFIG. 3. Diodes D1 and D2 serve as a means for providing a discharge pathfor coupling capacitors C1 and C2, respectively. Resistors R1 and R2serve to limit the current into the bases of Q1 and Q2 and prevent thelow impedance of the transistors from loading other circuits in thesystem. While a NOR gate is preferred, it should be apparent that anyother logical gate (e.g., AND, OR or NAND) could be used, so long as theoutput thereof is characteristic of the phase difference between thetransmitted and received pulses.

FIG. 4 depicts the output A0 of the phase comparator 13 for hypotheticalinputs 0 and 0 which, of course, are squared versions of the sinusoidaloutputs a and b of the transmitter and receiver elements, respectively.Note, the closer 0 and 0' come to being in phase, the wider the pulsewidth of A0 becomes, with the maximum pulse width being equal to halfthe period of oscillation of the transmitted wave. Conversely, thefurther 0 and 0 go out of phase, the narrower the pulse width of A0becomes, with the minimum pulse width being equal to zero.

The output A0 of phase comparator 13 is then fed to the input of thecircuit schematically illustrated in FIG. 5. This circuit includes theaveraging circuit 18, audio amplifier l9 and the Schmitt trigger 20, allof which, as previously mentioned being of conventional design. Theaveraging circuit, comprising resistor R5 and capacitor C3, serves toaverage its input, A0 so as to provide a fluctuating analog signal whenthe pulse widths of A** are fluctuating. As aforementioned, theinstantaneous frequency of the output of the averaging circuit isproportional to the rate of phase change.

The averaging circuit output is coupled to a conventional audioamplifier 19 via capacitor C4. This amplifier consists of transistors03, Q4 and Q5 and their associated bias resistors R6 through R17. Theamplifier is AC coupled so as to remove very low frequency components,such as those produced by air turbulence or the like, which mightotherwise overdrive the squaring circuitry that follows and therebyparalyze the detector for unacceptably long periods of time. Thecoupling capacitors C4, C5, C7 and C8 are pre-filters for the sharpdigital filtering which follows. Shunt capacitors C6 and C9 help toremove unwanted high frequencies which are completely eliminated in thefollowing digital gligr. Sensitivity is controlled by the adjustableresistor The output of the audio amplifier, shown in FIG. 6a, isreferenced to ground by the combined action of capacitor C8 and diode D3and this waveform is squared in the regenerative squaring circuit whichcomprises transistors Q6 and Q7 and their associated biasing resistorsR18 through R21. The squaring cir cuit, here a Schmitt trigger,functions by switching transistor Q7 from saturation to near cut-offwhenever the input at the base of Q6 exceeds the voltage drop acrossresistor R21. This voltage drop, constituting a noise rejection level,is determined by the ratio of the values of R21 to R20. The output ofthe audio amplifier at point V and the input to the output of theSchmitt trigger are shown in FIGS. 6A-6C, respectively.

The output of the Schmitt trigger is then processed by the digitalfiltering circuitry illustrated in FIG. 7. A low-pass filter, comprisingcapacitor C9, resistor R22 and diode D4, serves to reject all pulsesshorter than a predetermined value. Transistor Q8 and zener diode D5provide the threshold action for this filter. The filter is driven fromthe squaring circuit which acts as a low impedance switch whichimpresses rectangular pulses at the input to resistor R22. Upon receiptof a pulse, capacitor C9 begins to charge through R22 and the voltageacross C9 rises exponentially, as shown in FIG. 8. The voltage waveformacross C9, indicated as V in FIG. 8, is shown with a linear increasebecause it is near the start of the exponential charge which is cut-offlong before completion. As soon as the input pulse returns to zero,capacitor C9 discharges through diode D4. Thus, it is seen that forpositive voltages at the input, the digital filter acts as a simple RCintegrator with a time constant equal to R22 C9. The rapid dischargeprovided by D4, resets the integrator following each pulse. Referring toFIG. 8, it is apparent that narrow pulses from the squaring circuitresult in smaller peak voltages across C9 than do wide pulses. If apulse is wide enough to be passed, the voltage across C9 will exceed thethreshold established by zener diode D5, and transistor Q8 will conduct,thereby, switching transistor Q9 from cut-off saturation. Thus, onlythose pulses which are wider than a certain minimum value will betransmitted to the following high-pass filter. This type of circuit iscapable of rejecting two frequencies which differ by only a few cyclesper second out of many hundreds of cycles per second. Thus, it may beappreciated that extremely sharp low-pass filtering is provided.

Those pulses transmitted by the aforedescribed lowpass filter are thenfed to the high-pass filter input; i.e., the collector of Q9. Thehigh-pass filter essentially comprises a staircase counter of standarddesign. As 6 by diode D7 which is cut-off. Thus, each pulse receivedcauses the voltage across C11 to accumulate in steps. If resistor R26and transistor Q10 were absent, the charge on C11 would not decay andthe voltage across C11 would increase in steps until the total voltagereached approximately the input pulse magnitude. Because of the loadresistor R26, however, Cl 1 discharges between pulses. Therefore, if thepulses received are spaced far enough apart so that the voltage acrossC11 decays by an amount equal to the step increment generated by eachpulse, then no voltage accumulation will occur. If the pulse spacing isgreater, then the voltage across C11 will decay and if the pulse spacingis smaller, then the voltage across C11 will increase. (See FIG. 9).Resistors R27 and R29 form a voltage divider which sets a thresholdpoint for the conduction of transistor Q10. Resistor R28 is necessary toprovide a very high input impedance so that R26 is not loaded. If thepulse spacing is sufficiently close and enough pulses are received, thenQ10 will conduct and trigger the relay driver circuit to transmit analarm. Note that the high-pass filter requires not only a pulse spacingsmaller than a predetermined amount, but also a minimum number ofpulses. The circuits operation is weighted in favor of the highfrequencies as it takes more low frequency pulses than high frequencypulses to trigger Q10. This operation is equivalent to a weightedintegration which favors those frequencies located somewhat above thecut-ofi point.

From the foregoing, it is apparent that the high-pass filter performs alonger integration for those frequencies located near the lower edge ofthe bandpass and also for bursts of high frequencies with relativelylarge spacing. This characteristic is advantageous in that it improvesthe ability of the circuit to reject disturbances due to loud noises andmetallic impacts as might be caused by a dropped metal plate. Thesenoises are characterized by bursts of frequencies within the bandpassseparated by relatively long dead spots whereas a target looks far morecontinuous.

While the invention has been disclosed with particular reference to aphase-comparing Doppler system, it should be apparent that the inventionhas equal utility in the more conventional frequency-comparing system.To employ the invention in systems of the latter type, the Dopplerfrequency (i.e., the output of the mixer component) is amplified and feddirectly into a squaring circuit, such as the Schmitt trigger circuitshown in FIG. 5, and thereafter processed in the same manner asdisclosed above.

We claim:

1. A detection system for detecting motion of a particular character ina predetermined space, said system comprising:

a continuous wave transmitter for sending a wave of energy ofpredetermined frequency through the space wherein motion is to bedetected;

a receiver arranged to receive said wave of energy as modified by themotion of objects within said space;

a phase rate detector, operably coupled with said transmitter andreceiver, for comparing the instantaneous phase of the received wavewith that of the transmitted wave and for providing a digital outputsignal comprising a train of pulses having pulse widths proportional tothe rate at which the phase of said received wave changes with respectto said transmitted wave; and

digital filtering means, operably connected with said phase ratedetector output, for filtering out pulse widths uncharacteristic of theparticular motion to be detected.

2. A detection system according to claim 1 wherein said digitalfiltering means comprises high and low-pass filters connected in series.

3. A detection system according to claim 2 wherein said low-pass filteris fed by said phase rate detector output and comprises means fortransmitting only those pulses having a width greater than apredetermined value, and said high-pass filter is fed by pulsestransmitted by said low-pass filter and comprises means for counting thenumber of pulses transmitted by said lowpass filter and for providing anoutput signal by which an alarm may be activated when said countingmeans counts more than a predetermined number of pulses within apredetermined time interval.

4. A detection system according to claim 1 wherein said phase ratedetector comprises:

means, operably coupled with said transmitter and receiver, forcomparing the instantaneous phase of said received and said transmittedwave and for providing a digital output signal comprising a train ofpulses, each of said pulses having a pulse width proportional to theinstantaneous difference in phase between said transmitted and receivedwaves;

means for averaging said digital output of said phase comparing means soas to obtain an analog signal having a frequency substantially equal tothe rate at which the phase of said received wave changes with respectto that of said transmitted wave; and

means for converting said analog signal to a digital waveform comprisinga train of pulses, each pulse of which has a pulse width proportional tothe rate of change in phase.

5. The detection system according to claim 4 wherein said analog todigital converting means comprises a regenerative squaring circuit ofthe Schmitt trigger variety.

6. A detection system for detecting motion of a predetermined characterin a predetermined space said system comprising:

means for transmitting a continuous wave of energy of predeterminedfrequency through the space wherein motion is to be detected;

means for receiving said energy wave as modified by the motion ofobjects within said space;

a phase rate detector, operably coupled with said transmitting andreceiving means for comparing the instantaneous phase of the receivedwave with that of the transmitted wave and for providing a digitaloutput signal comprising a train of substantially rectangular pulseshaving a repetition rate corresponding to the rate at which the phase ofthe received wave changes with respect to the transmitted wave, saidphase rate detector comprising gate means, operably coupled with theoutputs of said transmitting and receiving means, for transmittingpulses having pulse widths proportional to the instantaneous differencein phase be ween said transrrutted and received waves, means oraveraging the output of said gate means so as to provide an analogwaveform having a frequency proportional to the rate at which the phaseof said received wave varies with respect to said transmitted wave, andmeans for squaring said analog waveform so as to provide said digitaloutput signal; and

digital filtering means, operably connected with said phase ratedetector output, for filtering out pulse widths uncharacteristic of theparticular motion to be detected.

7. A Doppler detection system for detecting motion of a particularcharacter occurring in a predefined space, said system comprising:

means for transmitting a continuous wave of energy of predefinedfrequency into said predefined space;

transducing means arranged to receive said wave of energy upon beingreflected and/or modified by objects within said predefined space, saidtransducing means being adapted to provide a first electrical signalhaving a frequency proportional to the frequency of the received wave ofenergy;

means for providing a second electrical signal having a frequencyproportional to the frequency of said transmitted wave of energy;

means for comparing said first and second electrical signals to providean analog signal of the Doppler frequency;

means, operatively coupled with said analog signal,

for providing a digital output signal comprising a train of pulses, eachpulse having a pulse-width proportional to the instantaneous Dopplerfrequency; and

digital filtering means for transmitting only those pulses in said trainof pulses having pulse widths within a predefined range of pulse widths.

8. The invention according to claim 7 wherein said wave of energy isacoustic in nature and said predefined frequency is ultrasonic.

9. The invention according to claim 7 wherein said digital filtercomprises low-pass filtering means for 50 transmitting only those pulseshaving a pulse width greater than a predefined value and meansoperatively coupled to said low-pass filtering means for counting thenumber of pulses transmitted by said low-pass filtering means and forproviding an output signal, 5 5 whereby an alarm can be activated, whensaid counting means counts more than a predetermined number of pulseswithin a predefined time interval.

10. The invention according to claim 7 wherein said squaring meanscomprises a Schmitt-type trigger.

1. A detection system for detecting motion of a particular character ina predetermined space, said system comprising: a continuous wavetransmitter for sending a wave of energy of predetermined frequencythrough the space wherein motion is to be detected; a receiver arrangedto receive said wave of energy as modified by the motion of objectswithin said space; a phase rate detector, operably coupled with saidtransmitter and receiver, for comparing the instantaneous phase of thereceived wave with that of the transmitted wave and for providing adigital output signal comprising a train of pulses having pulse widthsproportional to the rate at which the phase of said received wavechanges with respect to said transmitted wave; and digital filteringmeans, operably connected with said phase rate detector output, forfiltering out pulse widths uncharacteristic of the particular motion tobe detected.
 2. A detection system according to claim 1 wherein saiddigital filtering means comprises high and low-pass filters connected inseries.
 3. A detection system according to claim 2 wherein said low-passfilter is fed by said phase rate detector output and comprises means fortransmitting only those pulses having a width greater than apredetermined value, and said high-pass filter is fed by pulsestransmitted by said low-pass filter and comprises means for counting thenumber of pulses transmitted by said low-pass filter and for providingan output signal by which an alarm may be activated when said countingmeans counts more than a predetermined number of pulses within apredetermined time interval.
 4. A detection system according to claim 1wherein said phase rate detector comprises: means, operably coupled withsaid transmitter and receiver, for comparing the instantaneous phase ofsaid received and said transmitted wave and for providing a digitaloutput signal comprising a train of pulses, each of said pulses having apulse width proportional to the instantaneous difference in phasebetween said transmitted and received waves; means for averaging saiddigital output of said phase comparing means so as to obtain an analogsignal having a frequency substantially equal to the rate at which thephase of said received wave changes with respect to that of saidtransmitted wave; and means for converting said analog signal to adigital waveform comprising a train of pulses, each pulse of which has apulse width proportional to the rate of change in phase.
 5. Thedetection system according to claim 4 wherein said analog to digitalconverting means comprises a regenerative squaring circuit of theSchmitt trigger variety.
 6. A detection system for detecting motion of apredetermined character in a predetermined space said system comprising:means for transmitting a continuous wave of energy of predeterminedfrequency through the space wherein motion is to be detected; means forreceiving said energy wave as modified by the motion of objects withinsaid space; a phase rate detector, operably coupled with saidtransmitting and receiving means for comparing the instantaneous phaseof the received wave with that of the transmitted wave and for providinga digital output signal comprising a train of substantially rectangularpulses having a repetition rate corresponding to the rate at which thephase of thE received wave changes with respect to the transmitted wave,said phase rate detector comprising gate means, operably coupled withthe outputs of said transmitting and receiving means, for transmittingpulses having pulse widths proportional to the instantaneous differencein phase between said transmitted and received waves, means foraveraging the output of said gate means so as to provide an analogwaveform having a frequency proportional to the rate at which the phaseof said received wave varies with respect to said transmitted wave, andmeans for squaring said analog waveform so as to provide said digitaloutput signal; and digital filtering means, operably connected with saidphase rate detector output, for filtering out pulse widthsuncharacteristic of the particular motion to be detected.
 7. A Dopplerdetection system for detecting motion of a particular characteroccurring in a predefined space, said system comprising: means fortransmitting a continuous wave of energy of predefined frequency intosaid predefined space; transducing means arranged to receive said waveof energy upon being reflected and/or modified by objects within saidpredefined space, said transducing means being adapted to provide afirst electrical signal having a frequency proportional to the frequencyof the received wave of energy; means for providing a second electricalsignal having a frequency proportional to the frequency of saidtransmitted wave of energy; means for comparing said first and secondelectrical signals to provide an analog signal of the Doppler frequency;means, operatively coupled with said analog signal, for providing adigital output signal comprising a train of pulses, each pulse having apulse-width proportional to the instantaneous Doppler frequency; anddigital filtering means for transmitting only those pulses in said trainof pulses having pulse widths within a predefined range of pulse widths.8. The invention according to claim 7 wherein said wave of energy isacoustic in nature and said predefined frequency is ultrasonic.
 9. Theinvention according to claim 7 wherein said digital filter compriseslow-pass filtering means for transmitting only those pulses having apulse width greater than a predefined value and means operativelycoupled to said low-pass filtering means for counting the number ofpulses transmitted by said low-pass filtering means and for providing anoutput signal, whereby an alarm can be activated, when said countingmeans counts more than a predetermined number of pulses within apredefined time interval.
 10. The invention according to claim 7 whereinsaid squaring means comprises a Schmitt-type trigger.